Only disable -Wconstant-conversion for Clang, not GCC
Only disable -Wconstant-conversion for Clang, not GCC

The warning suppression was recently added in a51e912004a6, and I don’t
know what I was thinking. I went out of my way to make it apply to both
Clang and GCC, but GCC doesn’t recognize this warning at all, nor does
it need any other warning suppressed.

Change-Id: I50341bfe81ee4799b3f6278d2e31ec31741952ac
Reviewed-on: https://chromium-review.googlesource.com/755654
Reviewed-by: Joshua Peraza <f5809a5e35ec7e1bd0e25cb0a0b68bb433cf6f17@chromium.org>
Commit-Queue: Mark Mentovai <f1b5a91d4d6ad523f2610114591c007e75d15084@chromium.org>

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